1. Field of the Invention
The present invention relates to a method of producing a porous product and, more specifically, to a method that is capable of producing a porous product having a porosity as high as but not less than 50% by volume and controlling the porosity thereof by the use of a conventional injection molding machine or a conventional extruder. Further, the porous product produced according to the method of the present invention may be brought into direct contact with a human body.
2. Discussion of the Related Art
One known method of producing a porous product uses a molding material formed by mixing a resin with a volatile blowing agent such as carbon dioxide gas or ammonia gas, or a decomposable blowing agent such as azodicarbondiamide or dinitrosopentamethylenetetramine. With this method, the blowing agent is volatilized or decomposed in a heat-molding process to generate gas, which in turn forms pores.
In general, the method using a blowing agent for formation of pores produces porous products of the type having closed cells and, hence, is unsuitable for the production of porous products requiring a gas permeability. Also, such a method, in general, is prone to form individually larger pores or cells and, hence, is unsuitable for the production of porous products requiring higher functional characteristics such as cushioning characteristics or sound insulating characteristics. Further, since all of the blowing agent is not necessarily expanded, it is possible that some unexpanded blowing agent remains in the resulting porous product. Such residual blowing agent not only makes it difficult to estimate porosity of a resulting porous product but it also prevents the resulting porous product from being used in an application where the porous product is brought into direct contact with a human body, for example, as an ear plug, if the blowing agent contains ammonia, dinitrosotetamine or a like substance which is harmful to a human body.
A typical method of producing a form rubber comprises admixing of a blowing agent and a coagulant with a latex, and molding the resulting mixture. On the other hand, a typical method of producing a porous product such as urethane foam composes molding of a molding material formed by mixing a prepolymer with a blowing agent such as Freon or water as well as a catalyst for a curing reaction. These methods utilize a pore-forming mechanism relying upon the generation of gas volatilized from the blowing agent and, therefore, involve problems similar to those described above.
A method of producing a porous product such as an expanded polyethylene includes irradiation of a resin with an electron beam to cause the resin to be crosslinked while expanding a blowing agent. Although such a method is capable of controlling the porosity of a porous product, the electron beam irradiation results in an increased production cost. Besides, the problem of the residual blowing agent is still left unsolved.
A less costly method of producing a porous product enabling estimation and control of the porosity of a resulting porous product has been proposed. This method comprises the steps of: adding a powdery pore-forming agent such as sodium chloride or sodium sulfide (hereinafter referred to as a xe2x80x9csalt-type pore-forming agentxe2x80x9d) to a resin or rubber to form a molding material; subjecting the molding material to molding to provide a molded product; and washing the resulting molded product with water to elute the salt or the pore-forming agent thereby forming pores. Such a method is called a xe2x80x9cdesalting methodxe2x80x9d.
In a desalting method, since portions from which the pore-forming agent has been eluted become pores, and since the pore-forming agent itself is not expanded or foamed in the molding process, it is required that the molding material contain the pore-forming agent in an amount corresponding to the intended porosity. Accordingly, where a porous product having a porosity of 50% is to be produced, the pore-forming agent needs to be added to the resin component so that the volume ratio of the pore-forming agent in the resulting molding material assumes 50% or more by volume.
It is, however, difficult to mold such a molding material containing 50% or more by volume of the salt-type pore-forming agent. Specifically, the salt-type pore-forming agent remains in a solid state or powdery state at a typical molding temperature of resin because of its higher melting point. For this reason, the fluidity of a molding material decreases with a higher content of the salt-type pore-forming agent, and a molding material containing 50% or more by volume of the salt-type pore-forming agent fails to exhibit a fluidity (MFR value) required for molding. In injection molding or extrusion molding, in particular, the salt-type pore-forming agent in a powdery state cannot be sufficiently extruded or injected though the hydraulic resin component in the molding material that is to be extruded through an extrusion die or to be injected into a mold. This will result in a molded product containing the pore-forming agent in a lower amount, or in a non-homogeneous molded product containing the pore-forming agent which is rich in an inner part but lean or absent in a superficial part of the product, despite the pore-forming agent being abundantly contained in the molding material. Naturally, a decrease in the content of the pore-forming agent leads to a decrease in the porosity of a molded product. Further, such a non-homogeneous molded product gives a non-homogeneous porous product which eventually has a lower porosity than desired because the pore-forming agent is not sufficiently eluted by soaking due to the presence of too small an amount of the pore-forming agent in the superficial part of the molded product.
It is conceivable to improve the fluidity of the polymer component by raising the molding temperature. Even in this case, the salt-type pore-forming agent remains in a powdery state in the molding material and, hence, the pore-forming agent is difficult to extrude or inject as compared with the polymer component when the molding material is extruded through an extrusion die or injected into a mold, thus, resulting in a molding product containing a lower amount of pore-forming agent than desired.
An object of the present invention is to provide a method of producing a porous product which is capable of molding using a molding material containing an abundance of a pore-forming agent and controlling the porosity of an intended porous product as desired.
The inventors have studied a method which is capable of producing a porous product of which pores are individually minute and homogeneously distributed over the entirety of the porous product, and have found that the use of a pore-forming agent which assumes a solid state at room temperatures but can be melted into a liquid state at a molding temperature of a polymer component that will form the skeleton of the porous product, enables satisfactory molding without a decrease in the fluidity of a molding material as experienced in the prior art even when the amount of the pore-forming agent is increased in the molding material.
The present invention provides a method of producing a porous product which comprises the steps of: preparing a molded product by subjecting a molding material containing a polymer component and a pore-forming agent dispersed in the polymer component, the pore-forming agent assuming a solid state at a room temperature, to molding at a temperature which causes the pore-forming agent to melt; and soaking the molded product with a solvent which dissolves the pore-forming agent but fails to dissolve the polymer component, to form pores.
First, a molding material to be used in the present invention will now be described in detail.
The molding material comprises a polymer component and a pore-forming agent dispersed in the polymer component, the pore-forming agent assuming a solid state at room temperatures.
The pore-forming agent may be any compound that assumes a solid state at a room temperature and can melt at a molding temperature. Hereinafter, the pore-forming agent for use in the present invention will be termed a xe2x80x9cmeltable-type pore-forming agentxe2x80x9d for distinction from the salt-type pore-forming agent used in the prior art. The molding temperature varies depending on the polymer component to be used to form the skeleton of an intended porous product. As will be described later, the present invention uses polymer components that are generally moldable at about 100xc2x0 C. to about 300xc2x0 C. Accordingly, usable as the meltable-type pore-forming agent are compounds, preferably organic compounds, that melt at 100xc2x0 C. to 300xc2x0 C. Specifically, organic compounds having respective melting point between about 40xc2x0 C. and about 300xc2x0 C. are used. Preferably, such organic compounds are polyvalent alcohols.
If the melting point is higher than the above range, the molding temperature needs to be set considerably higher than the melting temperature of the polymer component, resulting in a danger of burning or thermal decomposition of the polymer component. Besides, this case uses energy wastefully. Although the salt-type pore-forming agent used in the prior art desalting method can melt into a liquid, the meltable-type pore-forming agent for use in the present invention characteristically melts at about the molding temperature of the polymer component. The salt-type pore-forming agent is an inorganic compound which has a higher melting point and, hence, as molding process requiring liquefaction of the salty-type pore-forming agent needs to be performed at a temperature considerably higher than the temperature which caused melting of the polymer component. In addition, the salt-type per-forming agent, even when liquefied by melting, is incompatible with the polymer component which is an organic compound and, hence, is difficult to disperse homogeneously in the polymer component. In contrast, an organic compound used as the pore-forming agent can readily be dispersed in the polymer component assuming a liquid state due to their superior compatibility with each other. This results in a homogeneous porous product.
Examples of specific organic compounds for use as the meltable-type pore-forming agent include polyvalent alcohols having 2 to 5 carbon atoms such as pentaerythritol, L-erythritol, D-erythritol, meso-erythritol, and pinacol; and urea. Among these, polyvalent alcohols, especially pentaerythritol, are preferable. Use of any such polyvalent alcohol makes it possible to use water as the solvent by virtue of its hydrophilic nature. As to pentaerythritol, it typically melts at 180xc2x0 C. to 270xc2x0 C. though depending on the purity thereof and hence provides a broader selection of the polymer component. Besides, since pentaerythritol solidifies rapidly, a resulting solid molded product can be cooled in a shortened time period, thus contributing to a higher productivity.
The meltable-type pore-forming agents described above may be used either alone or as a mixture of two or more of them. Where two or more of the meltable-type pore-forming agents are mixed, a combination of such materials has to be selected such that they have a substantially common melting temperature and can be eluted by a common solvent. When water is used as the solvent, an appropriate conventional salt-type pore-forming agent may be combined with an appropriate meltable-type pore-forming agent. From the standpoint of melting, however, a combination with any salt-type pore-forming agent is not recommendable.
The amount of the meltable-type pore-forming agent to be blended with the polymer component can be appropriately adjusted depending on the porosity of an intended porous product. Thus, according to the method of the present invention, the porosity can be controlled by adjusting the amount of the pore-forming agent to be used without any limitation imposed by the fluidity of a molding material, unlike the prior art desalting method. Accordingly, the method of the present invention is capable of producing a porous product having a porosity as high as 50 to 99% by blending the meltable-type pore-forming agent in an amount such as to assume a volume ratio of 50 to 99% by volume relative to the total volume of the molding material. With the conventional desalting method using the salt-type pore-forming agent, it is impossible to produce a porous product having such a high porosity by injection molding or extrusion molding. For the production of a porous product substantially free of any residual meltable-type pore-forming agent, the meltable-type pore-forming agent to be blended preferably amounts to 65% or more by volume in the molding material.
The polymer component for use in the present invention may be any polymer which can provide a molding material in a liquid state or can be molded in a liquid state. With such a polymer component capable of assuming a liquid state, it is possible to prepare a molding material having the meltable-type pore-forming agent substantially evenly dispersed therein, hence, to produce a porous product having pores evenly distributed over the entirety thereof. Examples of such polymer components that are capable of assuming a liquid state include thermoplastic resins and thermoplastic elastomers which can melt when heated, and thermosetting resins and rubber which can assume a liquid state at the beginning of molding and then be cured through crosslinking, such as prepolymers and liquid rubbers.
Examples of specific thermoplastic resins for use in the present invention include polyethylene, polystyrene, acrylonitrile (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, ethylene-vinylacetate copolymer (EVA) resin, ethylene.xcex1-eolefin copolymer, polypropyrene, polyamide 6, polyamide 6,6, polycarbonate, and polyoxymethylene (POM). Among these, preferable are those which are suitable for injection molding or extrusion molding. These polymer components may be used alone or as a blend of two or more of them in the molding material.
Thermoplastic elastomers, in general, comprise soft segments exhibiting rubber elasticity and hard segments forming knots of a three-dimensional network. Since such thermoplastic elastomers exhibit rubber elasticity at room temperatures and becomes plasticized at a higher temperature, they are suitable for injection molding or extrusion molding. Examples of specific thermoplastic elastomers for use in the present invention include: polystyrene elastomers having hard segments of polystyrene and soft segments of polybutadiene, polyisoprene or a hydrogenated one of these; polyolefin elastomers having hard segments of polyethylene or polypropyrene and soft segments of butyl rubber or EPDM (ethylene-propyrene-diene terpolymer); polyamide elastomers having hard segments of polyamide and soft segments of polyester or polyether; polyester elastomers having hard segments of polyester and soft segments of polyether; and polyurethane elastomers having hard segments of polyurethane block having a urethane bond and soft segments of polyester or polyether. These elastomers may be used either alone or as a mixture of two or more of them, or alternatively in combination with at least one of the foregoing thermoplastic resins.
Where the polymer component used in the present invention is a thermosetting resin, a liquid prepolymer, which is a precondensate, may be used as the polymer component and then caused to be cured through crosslinking in the molding process. Examples of such thermosetting resins include urethane prepolymers, unsaturated polyester resins, epoxy resins, novolac resins and resol resins. These may be used in combination with any appropriate curing agent, or allowed to be cured through dynamic crosslinking in injection or extrusion.
Examples of specific rubbers for use in the present invention include natural rubber, synthetic rubbers such as ethylene-propyrene-diene terpolymer (EPDM), butadiene rubber (BR), isoprene rubber (IR), styrene-butadiene rubber (SBR) and acrylonitrile-butadiene rubber (NBR), and liquid rubbers depolymerized to have a low molecular weight. Among these, liquid rubbers are preferable in terms of their applicability to injection molding or extrusion molding.
The proportion of the polymer component in the molding material can be appropriately adjusted depending on the porosity of an intended porous product. The method of the present invention is particularly useful for the production of a porous product having a porosity of not less than 50%. In case of the production of such a porous product, the amount of the polymer component in the molding material preferably ranges from 1 to 50% by volume, particularly 5 to 35% by volume based on the volume of the resulting molded product.
The molding material for use in the present invention may comprise, in addition to the foregoing polymer component and the pore-forming agent, an age registor, a plasticizer, a thermal stabilizer, a lubricant, a thickener, a flame-retardant, an antioxidant, a UV absorber, a coloring agent, an antistatic agent, a reinforcer or a like additive, as required. Preferably, such an additive is added in an amount of not more than 60 parts by weight based on 100 parts by weight of the polymer component. To a thermosetting resin or rubber may be added, as required, a crosslinking-causing compound for curing the polymer component such as a vulcanizer or vulcanization accelerator, or a curing agent, a crosslinking agent or the like, though such a resin or rubber can be cured through dynamic crosslinking or crosslinking based on heat reaction.
Where the polymer component is blended with the pore-forming agent and additives, the molding material is prepared by further dispersing the additives. Preferably, the preparation of the molding material is performed by kneading or mixing with use of a machine such as oven roll, kneader, intensive mixer, single screw extruder or twin screw extruder. Prior to such kneading or mixing, the components of the molding material may be premixed using such a mixer as Henschel mixer, twin-cylinder mixer, ball mill, ribbon blender or tumble mixer.
The molding material thus prepared is molded using a molding machine or an extruder to form a solid molded product. The temperature at which the molding is performed (molding temperature) is a temperature which allows the polymer component to be molded and causes the meltable-type pore-forming agent to melt. The temperature allowing the polymer component to be molded, though varying depending on the polymer component to be used, is a temperature at which the thermoplastic resin or elastomer melts, if they are used as the polymer component, or at which the rubber or the thermosetting resin can be cured through crosslinking, if they are used as the polymer component. Generally, such a temperature preferably ranges from 100xc2x0 C. to 300xc2x0 C.
The solid molded product may be formed by any process, for example, compression molding, transfer molding, injection molding, extrusion molding, blow molding, calendering and casting, without particular limitation. Among these, injection molding and extrusion molding are preferable in terms of their higher productivity. The injection molding is a process where a molten material plasticized and kneaded by a screw in a heating cylinder is injected with high speed and pressure into a mold shaped as desired and then solidified by cooling or by reaction to form a molded product. Since melting and kneading are performed in the heating cylinder of the injection molding machine, there is no need to previously prepare the molding material having the meltable-type pore-forming agent dispersed in the polymer component. Further, since the pore-forming agent used in the present invention assumes a molten state at molding, injection of the molding material can be achieved without separation of the pore-forming agent from the polymer component. Not only injection molding but also extrusion molding is preferably employed because it achieves melting-kneading and molding continuously and allows the molding material to pass through the extrusion die as having the pore-forming agent dispersed in the polymer component, thereby providing a homogeneous molded product.
The conditions for injection molding or extrusion molding may be appropriately determined depending on the polymer component and pore-forming agent used and their proportions. For instance, the feeding pressure and the injection speed can be determined as follows.
Generally, the number of revolutions of the screw is set to about 100 rpm so that the molding material is fed neither excessively nor insufficiently. If the rotary speed of the screw is higher than desired, the molding material may be fed insufficiently, resulting in entrapment of air and other disadvantages. For stable metering of the molding material, pressure is applied to a hydraulic cylinder when the screw operates. Such pressure preferably ranges from 5 to 10 kgf/cm2. The injection speed in a charging process may be adjusted to be higher when the intended molded product is thin, or to be lower when the intended molded product is thick. In a dwelling process, the dwelling pressure is set lower than the charging pressure. However, it is required that the dwelling pressure for forming a thin molded product is lower than that for forming a thick molded product because thin molded products are less susceptible to a sink (or sinkmark) that may be caused by solidification based on cooling as compared to thick molded products, which shrink or sink largely. For higher productivity, the mold temperature may be set lower to allow more rapid cooling. However, it is possible that such a lowered mold temperature causes the fluidity of the molding material to decrease thereby giving rise to charging deficiency or degrading the luster of surface of a resulting molded product. Thus, the mold temperature may be determined based on the polymer component in the molding material, taking such possible problems into consideration.
The molded product thus formed is soaked with a solvent that dissolves the meltable-type pore-forming agent but fails to dissolve the polymer component.
The solvent for use in the invention may be appropriately selected depending on the polymer component and pore-forming agent to be used. Examples of specific solvents include water, glycol, glycol ether, high-molecular-weight alcohol, fatty acid, fatty ester, glycol ester, mineral oil, petrol, alcohol ethoxylate, polyoxyethylene ester, glycerol, and glycerol ester. Since use of an organic solvent requires additional equipment for any post-treatment or the like, it is desirable that such a combination of the polymer component and the pore-forming agent be selected as to allow use of water as the solvent which does not require such equipment. Where a polyvalent alcohol is used as the meltable-type pore-forming agent, water can advantageously be used as the solvent.
The soaking process is executed by washing, dipping, immersing, or its equivalent with the above-mentioned solvent. The soaking process using the solvent causes the meltable-type pore-forming agent contained in the molded product to be dissolved in the solvent and eluted. In the soaking process, the meltable-type pore-forming agent that is present in the superficial part of the molded product is first eluted to form recesses, and then the solvent gradually penetrates deeper into the molded product from the formed recesses to elute the meltable-type pore-forming agent that is present in an inner part of the molded product. In this way, a product having individually minute pores leading to the outside of the porous product, namely an open cell type porous product, is obtained.
A porous product thus produced according to the method of the present invention may be used, for example, as an ear plug, shock absorber, heat-insulating material, filler, puff for cosmetic use, and filters. These applications depend on the polymer component used. Porous products obtained by the method of the present invention have an excellent gas permeability based on their many open cells and in addition, most of the pore-forming agent contained in a molded product is removed by the soaking process. Thus, such porous products can find applications, for example, as an ear plug or a cosmetic puff, which require hygienic qualities and safety for a human body. Further, the method of the present invention enables control of the porosity of an intended porous product by adjusting the amount of the meltable-type pore-forming agent to be blended in the molding material and is capable of providing a homogeneous porous product having pores distributed substantially homogeneously over the entirety thereof. Besides, such pores are individually minute. Thus, such a porous product obtained by the method of the present invention is superior to those obtained using conventional blowing agents in physical properties such as sound insulating properties, barrier properties and cushioning properties, and hence can advantageously be used as a high-performance filter.